Radio communication techniques have been developed remarkably in recent years. Various modulation methods such as the third generation mobile communication system CDMA, the OFDM expected as the mainstream of the fourth generation communication systems, etc. have been proposed so as to improve the frequency utilization efficiency (bit transmission rate per unit frequency) and some of those methods are already put to practical use. In principle, each of those modulation methods improves the frequency utilization efficiency by multi-value processings of information to be set in each of the baseband I and Q signals that are complex signals. As a result, the distribution of the I and Q signals has comes to be close to the normal distribution and the peak factor (also referred to as the crest factor) of an amplitude component represented by a peak-to-average power ratio with respect to an average power becomes a value as large as 10 dB or over.
Generally, in a power amplifier provided in the final stage of a radio transmitter, a trade-off relationship is seen between the linearity of amplification characteristics and the power efficiency. In other words, while an output of a power amplifier cannot exceed the saturation output determined by the transistor performance, the power efficiency reaches its peak in the output around the saturation. This is why the output of the amplifier is required to increase more to improve the power efficiency.
However, upon saturation of a modulated signal by the non-linearity of such amplification characteristics, a non-linearity distortion power leaks to another frequency band adjacent to the allowed transmission frequency band. The leak of this non-linearity distortion power comes to disturb other communication systems that use adjacent frequency bands. Thus such non-linear distortion power leaks to adjacent frequency bands are strictly regulated by the radio regulations and electric wave laws and regulations. In the case of CDMA, OFDM, or the like, however, modulated signals are easily saturated due to a peak factor including high amplitude components, thereby it cannot be expected to increase the power amplifier output as described above so much.
In such a situation, a peak factor reduction unit is expected as an effective means for solving the trade-off problem as described above. There are various methods proposed for such a peak factor reduction unit. Generally, the peak factor reduction unit controls each waveform so that the peak amplitude is limited within a predetermined value while allowing slight degradation of the waveform quality for the baseband I and Q signals. The following three points are assumed as indicators of the performance to achieve the peak factor reduction described above.
(1) The peak amplitude should be limited within a threshold value preset as an allowable range.
(2) The baseband spectrum should not be spread.
(3) The waveform quality should be less degraded.
For example, JP-A No. 2003-124824 (patent document 1) discloses a conventional technique that satisfies all those performance indicators.
Hereunder, the conventional technique will be described briefly.
FIG. 16 shows a simple functional diagram of a peak factor reduction unit proposed in the patent document 1. A complex signal including I and Q components and supplied as a digital signal from an input port i1 is branched to the first and second paths.
The band of the complex signal in the first path is limited by a baseband filter Hz0 having a transmission function H(z), then inputted to a correction signal generation unit 100.
In the correction signal generation unit 100, the complex signal is divided into an amplitude component and a phase component by a complex-to-polar coordinates conversion unit CP1, then output as an amplitude component sample sequence and a phase component sample sequence. The amplitude component sample sequence is inputted to a dead zone circuit DZ1 and an amplitude value (sample value) that exceeds a preset allowable range is detected. The sample value detected in the dead zone circuit DZ1 is multiplied by a predetermined gain in a gain block g0. This gain is set at a reciprocal of the maximum value of a coefficient (usually a tap coefficient center) of the baseband filter Hz0, thereby a sample value denoting a normalized exceeding amplitude value is obtained from the gain block g0.
The sample value output from the gain block g0 is inputted to a local maximum value detection unit 200 consisting of delay units D1 and D2, comparators LT1 and GT1, and a multiplier P1 respectively. In this local maximum value detection unit 200, the sample sequence output from the gain block g0 is branched into two paths and one of the branched sample sequences is delayed in the delay units D1 and D2 connected serially to each other so as to obtain three temporary consecutive samples one after another. The samples are an output sample of the gain block g0, an output sample of the delay unit D1, and an output sample of the delay unit D2.
The comparator LT1 compares the value in the delay unit D1 with an output of the delay unit D2 and outputs “1” for a period of “D2 output<D1 output” and “0” for other cases to the multiplier P1. The comparator GT1 compares an output of the gain block g0 with an output of the delay unit D1 and outputs “1” for a period of “D1 output>g0 output” and “0” for other cases to the multiplier P1. The multiplier P1 multiplies an output of the delay unit D1 by outputs of the comparators LT1 and GT1. Consequently, the multiplier P1 outputs an output value of the delay unit D1 as an impulse signal when the outputs of both the comparators LT1 and GT1 are “1”, that is, when the output sample of the delay unit D1 is a local maximum value larger than both preceding and succeeding samples.
The output of the local maximum value detection unit 200 (output of the multiplier P1) is supplied as an amplitude component signal to the polar-coordinates-to-complex conversion unit PC1. Because the processing delay of the signal in the local maximum value detection unit 200 is one sample, the phase component signal output from the polar-coordinates-to-complex conversion unit PC1 is also delayed by one sample in the delay unit D3 and supplied to the polar-coordinates-to-complex conversion unit PC1. The polar-coordinates-to-complex conversion unit PC1 converts both amplitude and phase components into a complex signal and supplied the signal to the subtraction unit SU1 as a correction signal used for peak factor reduction.
On the other hand, each input signal to the second path is delayed by a group delay having a transmission function H(z) of the baseband filter Hz0 in the delay unit D4, then supplied to the subtraction unit SU1 through the delay unit D0 having a delay time set in the correction signal generation unit 100, that is, a delay time of one sample generated in the local maximum value detection unit 200 in this example. The subtraction unit SU1 subtracts a correction signal output from the correction signal generation unit 100 from the complex signal output from the delay unit D0 and outputs a peak factor reduced signal. Then, the band of the signal is limited in the baseband filter Hz2, thereby obtaining a peak factor reduced baseband signal at an output port o2.
Here, the polar-coordinates-to-complex conversion unit PC1 outputs a correction signal (injected signal) generated for peak factor reduction. This output signal, when added to a complex input signal in the subtraction unit SU1 (minus addition), comes to cause degradation of the signal quality. However, because the correction signal is generated like an impulse and reaches its local maximum value when an amplitude component exceeds the allowable range, as well as because the energy is concentrated around the peak, the waveform quality degradation is minimized. In principle, the spectrum does not spread, since the band of the output signal of the subtraction unit SU1 is limited in the baseband filter Hz2.
In FIG. 16, Hz1 denotes a baseband filter provided to observe the waveform of an input signal of which peak factor is not reduced at the output port o1. Hz3 denotes a baseband filter provided to observe the waveform of an injection signal to be added to an input signal i1 for peak factor reduction at the output port o3. Those filters are not required necessarily for the peak factor reduction unit.
Hereunder, a simulation result will be described. The simulation is carried out with use of virtual parameters, that is, input I and Q signals assumed as complex random signals in accordance with the normal distribution of the sampling frequency 10 MHz, a cut-off frequency of each of the baseband filters H (z) (Hz0 and Hz2) as 4 MHz, and a peak factor limit value as 8 dB.
In FIG. 17A, a solid line denotes a frequency response of a baseband filter (z) and FIG. 17B shows an impulse response of a baseband filter. The parameters shown here are values set just for description; they do not mean those of a specific system.
FIGS. 18A and 18B show waveform amplitudes obtained in the simulation.
FIG. 18A shows a waveform of a signal of which peak factor is not reduced and observed at the output port o1 and FIG. 18B shows a waveform of a signal of which peak factor is reduced and observed at the output port o2. A broken line denotes an upper limit of an allowable range set in the dead zone circuit DZ1. In FIG. 18B, it is confirmed that the output port o2 outputs a signal of which amplitude is limited within a set value due to an executed peak factor reduction processing.
FIG. 19A shows a CCDF (Complementally Cumulative Distribution Function) for representing the frequency distribution of the peak signal. As seen in the spectrum shown in FIG. 19A, a correction signal observed at the output port o3 is masked completely by a signal observed at the output port o1 and not reduced in peak factor. Consequently, if this correction signal is used for peak factor reduction, the output port o2 can output a signal having no spread in its spectrum.
As seen in the CCDF shown in FIG. 19B, the signal observed at the output port o1 and not reduced in peak factor has a peak factor of 10 dB or over while the output port o2 outputs a signal of which peak factor is limited to about 8 dB. This means that execution of the peak factor reduction as described above makes it possible to increase the output to 8 dB of the saturation of the subject power amplifier, thereby the output can be increased by 2 dB even when the upper limit output from the original input signal is only 10 dB or under of the saturation output of the power amplifier. The waveform quality degradation in the above simulation is limited only to 1.3% in terms of the EVM (Error Vector Magnitude). Thus the quality degradation could be said extremely low.